The communication scheme to be a successor of W-CDMA (Wideband Code Division Multiple Access) and HSDPA (High Speed Downlink Packet Access), that is, long-term evolution (LTE), has been set forth by 3GPP, which is the standards organization of W-CDMA, and, for radio access schemes, OFDMA (Orthogonal Frequency Division Multiple Access) has been employed on the downlink and SC-FDMA (Single-Carrier Frequency Division Multiple Access) has been employed on the uplink. Presently, 3GPP is studying the successor system of LTE (LTE-Advanced (LTE-A) or Release 10).
The LTE system is a system to perform communication by sharing one, two, or a greater number of physical channels by a plurality of mobile stations UEs, on both the uplink and the downlink. A channel that is shared by a plurality of mobile stations UEs is generally referred to as a shared channel, and, in LTE, is a PUSCH (Physical Uplink Shared Channel) on the uplink or a PDSCH (Physical Downlink Shared Channel) on the downlink.
In a communication system using shared channels such as described above, to which mobile stations UEs the above shared channels are assigned needs to be signaled, per subframe, which is the transmission time unit. A subframe may be referred to as a transmission time interval (TTI).
In LTE, a PDCCH (Physical Downlink Control Channel) is defined as a downlink control channel to be used for the above signaling, and, furthermore, a PCFICH (Physical Control Format Indicator Channel) is defined as a control channel to report the number of OFDM symbols to be used for the PDCCH, and a PHICH (Physical Hybrid-ARQ Indicator Channel) is defined as a control channel to transmit ACK or NACK information for hybrid ARQ for the PUSCH.
The downlink control information that is transmitted by the PDCCH includes, for example, downlink scheduling information, UL scheduling grant, overload indicator, transmission power control command bit, and so on. Also, the downlink scheduling information includes, for example, downlink resource block assignment information, UE IDs, the number of streams, information related to precoding vectors, data size, modulation scheme, and information related to HARQ (Hybrid Automatic Repeat reQuest). Also, the above uplink scheduling grant includes, for example, uplink resource block assignment information, UE IDs, data size, modulation scheme, uplink transmission power information, and demodulation reference signal information.
The above PCFICH is information to report the PDCCH format. To be more specific, by means of this PCFICH, the number of OFDM symbols, to which the PDCCH is mapped, is reported as control channel format information (CFI: Control channel Format Indicator). In LTE, the number of OFDM symbols, to which the PDCCH is mapped, is one of 1, 2 and 3, and, in one subframe, the PDCCH is mapped from the top OFDM symbol.
On the downlink, a range to match the number of OFDM symbols (CFI value) reported by the PCFICH, from the top of the subframe, is the control channel region that is assigned to the PDCCH. The mobile station decodes the control channel region, and furthermore, if there is information for that mobile station, specifies and decodes the radio resource allocated to the PDSCH, based on the downlink control information.
On the other hand, in LTE-A, which is presently under study by 3GPP, there is an agreement to widen the band by gathering and grouping a plurality of component carriers. There might be a communication environment where some component carriers are subject to severe interference from other cells while other component carriers are not influenced much from interference. Then, a mechanism for assigning downlink control information (DCI), which relates to the shared data channels (PDSCH/PUSCH) transmitted by component carriers that are subject to severe interference from other cells, from other component carriers that are influenced less by interference, is under study. At the present stage, there is an agreement to add a bit field (CIF: Carrier Indicator Field) for setting a carrier indicator in downlink control information (DCI), reporting the presence/absence of the CIF from the base station apparatus (e-Node B), by RRC signaling, per mobile station (UE: User Equipment), and making the CIF three bits.